Acrylic syrups have been widely used in the continuous and cell casting arts. For example, Hellsund in U.S. Pat. No. 3,337,383 and Opel in U.S. Pat. No. 3,376,371 disclose the continuous casting of wide sheets of acrylic between two stainless steel belts. Other methods of production of acrylic products are well known in the art, and include processes such as mold and cell casting operations.
The addition of pre-formed particles is also known in the art. For example, Minghetti et al. in U.S. Pat. No. 5,530,041 disclose a process of making an acrylic-filled thermoformable acrylic sheet. There, pre-formed acrylic particles or chips are mixed into an acrylic syrup rich in monomer. The pre-formed particles comprise crosslinked acrylic. When mixed into the syrup, the particles tend to swell with monomer to at least 200% of their original volume. After the particle-filled syrup is cast and cured, the matrix is continuous throughout the finished sheet. That is, the matrix passes through the particles. The particles become part of the matrix, rather than a separate part of the finished sheet. This is in contrast to fillers such as polyester resin chips or glass spheres, which will not absorb monomer, and thus remain a distinguishable part of the finished product.
The above references disclose particles which are added in an amount wherein they occupy a significant volume of the pre-polymerization mixture. However, due to the geometry of the particles, there remains significant space between the particles. The present invention incorporates accent particles into these spaces.
The addition of accent particles is similarly known in the prior art. These particles may be soluble, partially soluble, or non-soluble in the matrix. Both soluble and partially soluble particles may tend to swell with monomer when added to the matrix. However, soluble particles will dissolve when added to the syrup. Therefore, their presence or absence is not important in the context of the present invention. The present invention specifically incorporates those type accent particles that are either non-soluble or partially soluble, and which do not swell with monomer to an extent that the integrity of the particle is lost.
Most choices for accent particles will have a specific gravity that is dissimilar to that of the acrylic matrix. Additionally, these particles are typically added in low volumetric amounts, in contrast to the pre-formed particles. This contributes to the uneven dispersion encountered with prior art compositions.
When accent particles are added to the syrup, they tend to migrate from the even dispersion obtained by initial mixing. This situation may be exasperated by the changes in viscosity of the syrup that are known to occur during the temperature increases that precede the polymerization phase of the operation. Additionally, when a continuous casting operation such as that described by Hellsund and Opel et al, is utilized the vibration and turbulence caused by the stainless steel belts tend to increase the rate of migration of the particles.
The results of accent particle migration in the matrix are detrimental to the finished product. If the particles migrate to either the top or bottom side of the casting belt, the finished product will experience aesthetic, as well as physical deficiencies. For example, if the particles comprise a non-thermoformable composition such as metallized polyethylene glycol teraphalate (PET), as is commonly used in boat hulls, the resulting sheet will be difficult to thermoform. The particles will tend to sink toward the bottom surface, and pack into place. This will prevent sufficient amounts of the matrix from reaching the lower portion of the sheet. Additionally, the resulting sheet will likely distort upon cooling, and may be brittle and tend to crack easily.
The present invention incorporates the above concepts, and comprises an improvement over these concepts. The present invention incorporates the use of pre-formed particles into the acrylic matrix for a novel purpose. The addition of these particles creates what is in effect a network that restricts the movement of other particles added to the syrup. The result is unexpected in view of the prior art. The products of the present invention have a relatively even dispersion of accent particles throughout the finished product. These particles will be relatively evenly distributed even if they possess a specific gravity that is dissimilar than that of the acrylic matrix. Additionally, the orientation of these accent particles will tend to be random, rather than uniform, throughout the finished product. This is particularly advantageous when the accent particles used have an asymetric three dimensional geometry, such as flat particles.